Since my last post with image captures of the half finished room's freq. spectrum, more progress has been made. Over 90% of the rooms internal surfaces have been lined with the blue fill, with the exception of the floor. A second door, a solid core, has been hung, and weatherstripping has been applied all around the perimeter of the door. With both door closed, the sound proofing is pretty good, although the wall with the door faces the liveliest (acoustically) adjacent room, and still remains the most "porous" of the room's walls. A slot to run the mic cable into the room had to be cut in a new footer for the door, which is currently too tight -- the doors have to be force-closed.

View from the door. The section to the right of the lamp is where the window used to be. Thus far, about 560lbs of the blue fill is in the room.

View from the opposite side -- door and portion of that wall is still unlined.

On this Friday, at 8:30 am with typical traffic morning rush hour traffic on Main St 1.5 blocks away, the A-weighted SPL in the room was at a steady 12 dB. From the middle of the front yard, without any audible planes or immediate traffic, the SPL is around 26~28 dBA, with some 2-6 dBA jumps in level when large vehicles pass the intersection of my avenue and Main St., to which there is a line of sight. Such jumps in level are not registered by the mic in the test chamber.

Traffic directly in front of the house (no big trucks so far) causes the SPL to rise 1-2 dBA, and traffic at the nearest intersection half a block away, where cars accelerate from a stop, seems to cause only about a 1 dBA increase from the baseline 12 dBA in the lab.

Airplane noise seems to affect the overall level by no more than 1 dBA, but there have not been any very close planes so far today.

All the rises in noise are audible through the Grado SR60 headphone used for monitoring, but only when recording and playback gain is at maximum. With most of the sounds, it's just a bit of added low freq rumbling; with some sounds, a bit of the midband comes through -- but it's at a very low level. A single quiet fan in the room is probably enough to mask all this.

Will the noise level be improved when the room is 100% done?

Yes.

A portion of the door-wall is unlined, and as mentioned, this is the weakest wall -- along with the doors, which are also not lined. I'm sure some sounds come straight through the doors. Once these are treated, I expect the overall noise will drop at least 1 dBA, and spurious noise rejection will improve several dBA. The ceiling is currently lined with just one 8" layer of blue fill; it can be doubled up later if deemed necessary.

Keep in mind our objectives:

1) lower ambient than the quietest gear we measure (already achieved)
2) elimination of sonic reflections in the room (mostly achieved to below 300Hz)
3) freedom from external noise interference -- this is really the trickiest part. Do we have enough freedom? So far, it seems borderline. Once everything outlined above is done, I think it will be good enough. Most of the noise that comes through is low freq (below 200Hz) and even the room-in-room construction will not help much without a floating mass loaded floor, which is not an option I'm willing to pursue at this time.

RE: The door: This will sound a bit crazy and might be no use at all, but have you considered putting a layer of lead on the inside/outside of the door, you would of course have to line it with rubber first, and add a few hinges. Would it help at all with the noise.?

Looking at normal doors, they seem to be 78x30 inches, which is 1.98m x 0.76m. This is 1.5 M2, which would weigh in at a tiny 30KG's if 1.8mm thick sheeting is used.

A little early for congrats yet, I'm afraid. Need to finish -- and spend probably a week in testing before making a final judgement on whether the room is done.

Lead is not a great choice because of exposure to human contact -- it's the density and weight that makes it good, but on doors that are hung, it might be too much weight for the hinges. I plan to line one or both sides of the inner hollow core door with 1/2"~3/4" MDF before hanging blue fill on the inside. I'm figuring out a simple way to keep the coverage of blue fill constant over the door and hinged area.

Russ pointed me to an automatic door bottom -- a device which seals the gap under the door when it's closed. The trick is to find one in Vancouver.

Can you replace the hollow-core door? Wouldn't a second solid door help?

Yes, but it's a pain and time-consuming. Getting one, to begin with, and then the details of door mounting on an existing frame. Less of a pain to just line it with MDF -- and probably about equally effective, especially with some rubbery gasket or sheeting between.

Russ pointed me to an automatic door bottom -- a device which seals the gap under the door when it's closed. The trick is to find one in Vancouver.

Is it possible to seal the door manually from the outside every time you run tests?

Actually, even as it stands now, the seal at the bottom is very good, and it can be made excellent just by adding a stop with gasket atop the tight fitting footer. What an auto door bottom provides is greater convenience -- and a smoother floor since the footer can be lowered or replaced with one that has smoother contours and lower height. The main advantage of that will be in moving the PSU tester, which is on a stand with wheels, in/out of the room. If Pemko does not get back to me in a reasonable time, I'll just go with a stop and gasket for both doors -- since they close from opposite directions, one stop of the right dimensions with gaskets on both sides will work.

Sunday July 20 -- There's usually less traffic on Sunday, so I tried more SPL measurements of the room, which has changed little since my last post.

On the advice of the creator of the SpectraPLUS spectrum analyzer software I'm using, the FFT (Fast Fourier Transform) size was increased to improve resolution at lower frequencies. The FFT size in previous measurements had been set to 4096 PTS, which resulted in a sampling resolution of 21.5Hz. Increasing the FFT size to 65536 gave a much higher resolution of 1.35 Hz. It changed the curves below ~500Hz.

This was captured at 4pm Sunday June 20 with the FFT at 4096, flat freq response -- without weighting.

This was captured a minute or two later with the FFT at 65536, flat freq response -- without weighting.

Comparing the above images, it's clear that the ~60Hz peak in the second higher resolution curve comprises much of the elevated response in the sub-100Hz range of the first image. That 60Hz peak may be coming from the hum of one or more AC motors in the fridges in the house... or it could be a room resonance... or it could be both. Again, more experimentation is needed to get at exact causes.

A similar phenomenon can be observed in the range between 300~400Hz. The first image shows a broad rise; the second image shows that it's really a very narrow spike at ~370Hz. Moving the mic and/or table in the room, and also finishing the blue fill lining in the room could affect such spikes.

This was captured a minute or two later with the FFT at 65536, with A-weighting applied.

In conclusion, it's obvious that the high FFT setting is much more useful in examining details of the room's acoustics, and more work will be done with this in the near future.

The US electrical network uses 60 Hz, doesn't it? If so it seems pretty likely that a motor is the cause. If you try a opule of diffrente microphone positions, you shuld be able to see if its a room resonance i guess.

Have you considered using visco-elastic glue or other such products for panels when lining things (like doors) or perhaps on the floor?
A correctly implemented visco-elastic panel absorbs 8 - 16 db worth of vibration. This technology is frequently used in high end speakers, vehicles, buildings where acoustics is a priority etc.
This will be useful for the difficult lower part of the spectrum.

I suggest you replace any light weight door with a half-hour fire check door (preferably two in separate frames). They're are commonly used in offices and I've seen them used to good effect in studio's (where the cost of lead lined doors was prohibitive)

I suggest you replace any light weight door with a half-hour fire check door (preferably two in separate frames). They're are commonly used in offices and I've seen them used to good effect in studio's (where the cost of lead lined doors was prohibitive)

BTW impressive results so far

There are also doors that are designed to keep the noise out. You should use 2 and fill the gap between them with noise canceling material (stick it to one door and make sure its a tight fit).

A summary, after the past week's experimentation with the mostly treated room.

It now measures ~12 dBA pretty much any time of day or night.

There are some spikes in the response curve most likely related to reflections off the mic stand, the table, the wood frames for the blue fill, etc, but these are minor issues.

The acoustic isolation is not as good as hoped for. When it's reasonably quiet outside, it's perfectly quiet in there. This could be said before any of the treatment was done... but the ambient was some 2-4 dBA higher. When there's plane/car noise, it's at a much lower level than before, but still audible.

Would any of this noise come through in the recording?
A bit, with the quietest products, if you turn up the playback gain high enough. But if you follow the directions about how best to listen to them (which is to turn the volume to the point where the ambient turns inaudible), then no, you'd never hear it.

What about in SPL measurements?
Not really, not with A-weighting.

At the start of the construction, when I first discovered the dramatic effect of sealing up that window and doubling up the external wall, I asked whether this was good enough sound isolation for SPCR's needs -- and everything I've done with the room since then has been to try and answer this question.

The answer is both yes and no.

Yes -- it's probably good enough, given the objectives: Higher accuracy and greater ease in making recordings and measurements of quiet gear for SPCR.

No -- not if we want to be able to ignore outside noise altogether. It probably could be made 10 dB quieter in the lower freq... maybe better, quiet enough to be a "serious" anechoic chamber.

What would this take?
Going back to the original design -- a room in a room -- but adding one more feature: a floating floor of 2-3 layers of 3/4" plywood on neoprene damping pucks. The walls would have to be built atop this floor, making no contact with any of the existing walls. The ceiling would bolt atop the walls w/o touching the existing ceiling. As mentioned in another thread, it's been done by a Canadian digital recording tech store and at least partly documented.

All this would mean undoing much of the work I've done. The nearly-600lbs of blue fill would have to go back into the garage, the wire suspension on the ceiling removed, etc. But this is relatively simple, compared to the original installation. Re-installation of the blue fill would go much more smoothly now that I've figured out how, and w/all the frames for the blue fill already built. Still it would mean the mess down here continues on, and even get worse with drywall dust being added to the mess, for at least a couple more weeks.

Will I do this? At present, I'm torn. PArt of me just wants to finish off what I've done already and get on with more and better reviews & articles. Another part wants to make the chamber the best that it can be -- and I'm half way there. Decisions, decisions.

More later.

------------------------

An Aside: Late last night, I had the mic in the room with the doors sealed shut, and studying the freq spectrum on the screen as I listened through headphones. This was to understand better the relationships between what I hear, what the mic picks up, and what shows up in the RTA screen. It would help identify the sounds that are getting through to the mic in that room.

It was very quiet, around midnight on a Sunday. The gain was turned up to max -- what I heard in the bass through the mic was louder than what I could hear with my bare ears, even though I was outside the treated room. Furthermore, because the rest of the freq range was so subdued, the low freq sounds came through much more clearly.

There were numerous little spikes in the sub-100Hz region, and I was trying to identify these sounds. Some were obvious, like a car a block a way, idling while dropping someone off, the entry door being closed a few houses away, etc. All these were still registering under 20 dB. But there was a series of rhythmic thumps, perhaps 15 in total with a slight jog in the middle, that I could not identify immediately... until I heard them again in a slightly different rhythm a few minutes later. The second set of thumps made me realize it was someone coming back down a staircase with a landing in the middle, after having gone up the stairs a few minutes before. Thinking through, it was probably the next door neighbor. The outer wall of that house starts about 10-12 feet from the lab room window. I have never heard neighbors walking up/down their stairs before.

Thankyou, Mike. You're going to a crapload of trouble here, just to give us better testing for the reviews. I for one, appreciate the hell out of it.

Was going to make a construction suggestion here.. then realized after I got to about the third paragraph, that you have interior assembly mostly finished at this point. Unfortunately, it didn't occur to me earlier.

The good news is, when the chamber isn't busy, you can use it for music/movies/etc. Perhaps a portable setup on a rolling cart that you could move out of the room when you're doing testing

Take the time to calculate the volume of air in the room, and how long it can sustain a human being/beings over time without becoming 'fouled'...

If you're going to close the door, make sure you set a stopwatch or something to remember to open the door.... anoxia (if I'm remembering the term correctly?) can easily sneak up on you and render you.... unable to open the door :/

Forgive me if I'm being a bit paranoid. I'd hate to hear you'd harmed yourself accidentally just trying to give us better testing. Safer is better!

On the spectrum analysis chart with the higher resolution it looks like some of the peaks are multiples and halves -- peaks at 45hz, 60, 75, 90 (sorta), 120, 300, 360, 480, 600, 720, 840. Curiously missing is 240 and 960. Could mean something or not.

More importantly, I second TheAtomicKid's excellent suggestion. I've seen the devastating effects of hypoxia. It can take a cumulative toll without any obvious crisis.

Both doors now shut and seal properly. Identified three sources of hum in the room: An air purifier, our fan speed controller, and the tiny AC/DC adapter for the mic's 200VDC power source. Only the mic adapter was actually turned on, the others just plugged in. Unplugged all three and turned off the single CFL bulb in there, switched to a 9V battery for the mic power source.

The SPL is now about 10 dBA. The occasional jet plane pushes this up as high as 15 dBA, and registers as rumbling <100Hz.

I worked in the recording industry for 30 years and been involved with the construction of a few rooms (although none were anechoic or designed with the noise floor that you're looking for).

Some random thoughts;

Room in room:

This will expensive and difficult to do properly. You'll need to consult someone about the room suspension, since it's critical and has to be the first thing in place.

How about building a suspended test chamber. i.e. something much smaller, just big enough for item under test ( ISTR B&K used to make a spherical anechoic isolation chamber, about a couple feet in diameter, for testing mics)

Low frequency noise:

Do you need to worry about this at all? Little of the equipment you test is likely to generate significant levels of LF noise, why not just add an additional, say 200Hz, high pass filter to your results?

Your spectra shows 60Hz plus harmonics. How is the mic amp connected to the PC (i.e. liberal use of isolating transformers might be a help) .

Do you, or anyone within a hundred yards, have a constant voltage transformer? (these things radiate an enormous magnetic field that can be a real **** to to get rid-of)

P.S. to get the best out of your mic suspension you have to carefully dress the cable - the one in the pics looks far too tight

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